The purpose of this study is to characterize the nature and biological significance of cell-specific tropism observed in HIV-1 infections. Persistent infection in monocytic cells is thought to be a mechanism of latency in lentiviral diseases. It is possible that HIV-1 infection of monocyte lineage is responsible for persistent infection and may be the principal mechanism for cell-to-cell transmission of HIV-1. Three sets of paired viral strains with established lymphocyte and/or monocyte-tropic growth capabilities have been isolated. These isolates have been cloned and sequencing of the 3' end of the genome is underway, including ENV., NEF, and the upstream portion of the 3' LTR. Areas found to be unique to one biologically distinct isolate will be further analyzed for ability to confer cell-specific tropism. Recombinant strains of HXB2 and the 3' ends of our various paired clones will be analyzed for cell specific tropism on cultures of fresh peripheral blood monocytes and lymphocytes. If the recombinants demonstrate that replicative capability in monocytes is controlled by the 3' end of the genome, smaller fragments will be subcloned to identify the region responsible for this selected growth. Further characterization of factors enhancing growth in monocytes will performed, including growth factors, viral promoters and agents used for cell differentiation. To further characterize the possible contribution of the 3' LTR for activation in monocytes, LTR-CAT expression vectors representing all available paired clones will be constructed, transfected into cell cultures and analyzed for activity in different cell types. Preliminary constructs demonstrate that the LTR's derived from monocytetropic clones confer greater activation of CAT activity in the monocyte cultures than the construct containing the LTR from a "lymphocyte-tropic" clone. Methods to improve transfection efficiency and optimize culture conditions are currently being investigated. The effect of growth factors on LTR activation will also be pursued. If the 3' end of the genome does not confer cell specific tropism, the 5' end will be cloned and used in the above strategies. The above strategies should provide several mechanisms to define the HIV-1 genomic area responsible for cell-specific tropism, and other factors that regulate infection and replication in a given host cell.